A conventional rod lens assembly comprises a small-bore flexible tube containing a series of cylindrical rod lenses maintained in axially-spaced relation by small cylindrical sleeves or spacers positioned between the lenses, as illustrated, for example, in U.S. Pat. No. 3,257,902. Even where the wall thickness of the spacers is kept to a minimum and care is taken to insure that the annular ends of those spacers contact the lenses only at their outer edges, such spacers nevertheless significantly reduce the light and image transmitting properties of the lenses by rendering portions of their end faces optically inoperative. In addition, flexure of such an assembly in use may cause the spacing sleeves to abrade or damage the edges and end faces of the lenses, not only resulting in the creation of objectionable debris but also causing possible changes in the critical distances between adjacent lenses. While the risk of damage to the lenses might be reduced by increasing the wall thickness of the spacers, so that they engage larger portions of the end surfaces of the lenses, such a modification is otherwise self-defeating because of further unacceptable reductions in light and image transmission.
An even more serious problem inherent in the construction of a typical rod lens system as so described is the possibility that flexure of the assembly may result in transverse fracture of one or more of the glass lenses, thereby rendering the entire system inoperative and requiring either costly and time-consuming repair or total replacement of the entire assembly. The seriousness of the problem is compounded by the fact that an endoscope containing such a rod lens system is commonly used in conjunction with working elements, deflecting bridges, grasping forceps, lithotrites, and other relatively heavy surgical or urological instruments which, if brought into forceful accidental engagement with the lens-containing tube of such an endoscope, might easily cause flexure of the tube and fracture one or more of the lenses.
The use of spacers to set the spacing between successive rod lenses of a series during assembly thereof has the further disadvantage that even a slight variation from the optimum length of each sleeve could result in a tolerance buildup which might substantially impair the optical properties of the final product. Conversely, the almost-microscopic adjustments in the axial positioning of the lenses which might be needed to provide the best optical performance for the lens train as a whole would be extremely difficult if not virtually impossible to achieve in a system where sleeves must be inserted to set the spacing between successive lenses of the series.